Hockeydad is right.
There was a massive exodus from the Pittsburgh area in the late 70s, early 80s as the mill economy began to dissolve.
The fans took their loyalty along with them and cultivated it in their families. The team image is also appealing to those who value hard work, integrity, and professionalism.
There MAY be a massive nationwide following for Dallas as well, but that is mostly due to the appeal of the team to ****, criminals, and urbanites.